The demand for iso-alkenes has recently increased. For example, relatively large amounts of isobutene are required for reaction with methanol or ethanol over an acidic catalyst to produce methyl tert-butyl ether (MTBE) or ethyl tert-butyl ether (ETBE) which is useful as an octane enhancer for unleaded gasolines. Isoamylenes are required for reaction with methanol over an acidic catalyst to produce tert-amyl methyl ether (TAME). With passage of the Clean Air Act in the United States mandating increased gasoline oxygenate content, MTBE, ETBE and TAME have taken on new value as clean-air additives, even for lower octane gasolines. Lead phasedown of gasolines in Western Europe has further increased the demand for such oxygenates.
An article by J. D. Chase, et al., Oil and Gas Journal, Apr. 9, 1979, discusses the advantages one can achieve by using such materials to enhance gasoline octane. The blending octane values of MTBE when added to a typical unleaded gasoline base fuel are RON=118, MON=101, R+M/2=109. The blending octane values of TAME when added to a typical unleaded gasoline base fuel are RON=112, MON=99, R+M/2=106. Isobutene (or isobutylene) is in particularly high demand as it is reacted with methanol to produce MTBE.
The addition of shape-selective zeolite additives such as ZSM-5 to cracking catalysts, e.g., those used in fluidized catalytic cracking (FCC), is beneficial in producing gasoline boiling range product of increased octane rating. However, increased amounts of olefins result, including n-butenes, creating a need for their conversion to higher value products such as isobutene which can be used to produce MTBE.
Butene exists in four isomers: butene-1, cis-butene-2, its stereo-isomer trans-butene-2, and isobutene. Conversions between the butenes-2 is known as geometric isomerization, whereas that between butene-1 and the butenes-2 is known as position isomerization, double-bond migration, or hydrogen-shift isomerization. The aforementioned three isomers are not branched and are known collectively as normal or n-butenes. Conversion of the n-butenes to isobutene, which is a branched isomer, is widely known as skeletal isomerization.
The reaction of tertiary olefins with alkanol to produce alkyl tertiary alkyl ether is selective with respect to iso-olefins. Linear olefins are unreactive in the acid catalyzed reaction, even to the extent that it is known that the process can be utilized as a method to separate linear and iso-olefins. The typical feedstream of FCC C.sub.4 or C.sub.4 + crackate used to produce tertiary alkyl ethers in the prior art which contains normal butene and isobutene utilizes only the branched olefin in etherification. This situation presents an exigent challenge to workers in the field to discover a technically and economically practical means to utilize linear olefins, particularly normal butene, in the manufacture of tertiary alkyl ethers.
In recent years, a major development within the petroleum industry has been the discovery of the special catalytic capabilities of a family of zeolite catalysts based upon medium pore size shape selective metallosilicates. Discoveries have been made leading to a series of analogous processes drawn from the catalytic capability of zeolites in the restructuring of olefins.
European Patent 0026041 to Garwood, incorporated herein by reference, discloses a process for the restructuring of olefins in contact with zeolite catalyst to produce iso-olefins, followed by the conversion of iso-olefins to MTBE and TAME. The restructuring conditions comprise temperature between 204.degree. C. and 315.degree. C. and pressure below 51 kPa.
In European Patent 0247802 to Barri et al., it is taught that linear olefins can be restructured in contact with zeolite catalyst, including Theta-1 (ZSM-22) and ZSM-23, to produce branched olefins. The restructuring conditions comprise temperature between 200.degree.-550.degree. C., pressure between 100 and 5000 kPa and WHSV between 1 and 100. Selectivities to isobutene up to 91.2% are reported using a calcined Theta-1 tectometallosilicate at 400.degree. C. and 30.6% 1-butene conversion.
U.S. Pat. No. 3,992,466 to Plank et al. teaches the use of ZSM-35 as a catalyst for hydrocarbon conversion reactions, including "isomerization of aromatics, paraffins and olefins."
U.S. Pat. No. 4,922,048 to Harandi discloses the use of a wide variety of medium pore size zeolites, e.g., ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-35 and ZSM-48, in low temperature (232.degree.-385.degree. C.) olefin interconversion of C.sub.2 -C.sub.6 olefins to products including tertiary C.sub.4 -C.sub.5 olefins and olefinic gasoline.
U.S. Pat. No. 4,886,925 to Harandi discloses low pressure high temperature conversion of light olefins to produce higher olefins rich in isoalkenes. The process converts C.sub.2+ n-alkenes to a product comprising C.sub.4 -C.sub.6 alkenes rich in iso-alkenes, C.sub.7 + olefinic gasoline boiling range hydrocarbons, and unconverted hydrocarbons over ZSM-5. The reference teaches further treatment of the alkene effluent with methanol in the presence of medium pore size zeolites such as ZSM-5, ZSM-11, ZSM-12, ZSM-35, ZSM-38 and ZSM-48.
U.S. Pat. No. 4,996,386 to Hamilton, Jr. discloses concurrent isomerization and disproportionation of hydrocarbon olefins using a ferrierite/Mo/W/Al.sub.2 O.sub.3 catalyst. The catalyst exemplified produces fewer branched olefins than a comparable material free of ferrierite and the reference teaches that ferrierite-containing catalysts exhibit improved selectivity to linear olefins than conventionally prepared disproportionation catalysts.
All of the above references are incorporated herein by reference.
Despite the efforts exemplified in the above references, the skeletal isomerization of olefins e.g., to produce isobutene, has been hampered by relatively low conversion and/or selectivity to isobutene perhaps owing to the lability of these olefins. It is further known that skeletal isomerization becomes more difficult as hydrocarbons of lower molecular weight are used, requiring higher temperatures and lower linear olefin partial pressures.
Generally, the conversion of n-butenes to iso-butene is conducted at selectivities below 90%. In order to obtain higher selectivities, operation at high temperatures (&gt;500.degree. C.) and with high feed dilution (butene partial pressure, typically less than 5 psia (34.5 kPa)) is generally required. Selectivities of greater than 85%, 90%, 95% or even 99% are highly advantageous in commercial conversion of n-butenes to isobutene in order to avoid the need to separate out materials other than n-butene from the product stream. Such high selectivities will permit direct (cascading) or indirect introduction of the isomerizer effluent to an etherification zone where isobutene is reacted with alkanol to produce alkyl tert-butyl ether, e.g., MTBE. Unconverted n-butenes in the isomerizer effluent can be withdrawn either before the etherification zone or preferably, from the etherification zone effluent insofar as the etherification reaction utilizes only the isobutene component of the isomerizer stream. Unreacted n-butenes from the etherification zone effluent can be recycled to the isomerizer where they are converted to isobutene at high selectivity. If the recycle stream contains not only unconverted linear olefins, e.g., n-butenes, but also by-products such as other olefins (e.g., propylene) or paraffins, they have to be removed from the recycle stream, such as by distillation or by taking a slip stream. These removal steps are expensive and can lead to considerable loss of not only the by-products but butenes as well. These losses are larger when the by-products formed are present in higher concentration. Thus, even small improvements in the isobutene selectivity during n-butene isomerization have a major effect on the commercial viability of the process.
Further enhancement of total yield of iso-olefin can be effected by enhancing overall conversion of the n-olefin-containing feedstream. With this object in mind, it would be advantageous to provide a skeletal isomerization catalyst capable of maintaining a high level of conversion as well as high iso-olefin selectivity, even at relatively low temperatures, e.g., no greater than 450.degree. C. and high n-olefin space velocities. ZSM-35 catalysts comprising zeolite loadings of 50 wt % or higher and inorganic oxide binder have been found useful for this process. However, such catalysts have relatively high activity and therefore require relatively high linear olefin WHSV operation, e.g., (30-60).
Skeletal isomerization of linear olefins has been characterized as being favored at relatively low linear olefin partial pressures, e.g., less than one atmosphere. This can be done by diluting the linear olefin-containing feed with a suitable diluent, e.g., nitrogen. In order to provide a commercially acceptable overall conversion rate, other process parameters can be adjusted, e.g., increasing the space velocity of the diluted feed to the reactor. However, increasing space velocity to a level sufficient to provide acceptable overall conversion can bring on attendant high pressure drops which can create additional problems, especially where the olefin skeletal isomerization catalyst selectivity to iso-olefins is sensitive to changes in pressure, as is the case with catalyst comprising relatively high levels of ZSM-35.
Accordingly, it would be useful to find a way of reducing olefin space velocity in order to minimize pressure drop effects. It would also be useful to provide an olefin skeletal isomerization catalyst which is relatively insensitive to changes in linear olefin partial pressure caused by reactor pressure drops, which would allow operation at higher space velocities and a broader range of linear olefin partial pressures resulting in a larger operating window.